A four-year-old girl in Italy has died of malaria, raising fears over the comeback of the mosquito-borne disease decades after it has been eliminated in the country.

Sonia Zago developed a high fever after returning home from a vacation in Bibione, a seaside town on the Adriatic coast near Venice.

As her condition worsened, she was taken to a hospital in the northern city of Trento. Within an hour of arriving, the girl had fallen into a coma and was diagnosed with malaria. Zago was then transferred to a hospital in Brescia that treats tropical diseases, but she died on the night of September 3.

The girl had never traveled to a risk-prone country, raising questions about how she contracted the disease.

The Ministry of Health confirmed on September 5 that it had dispatched a team of experts to investigate.

Dr. Claudio Paternoster, director of the infectious diseases ward at Santa Chiara hospital in Trento, said that he had not seen a case of home-grown malaria during his 30-year career.

He said that Zago had been admitted to the facility about two weeks ago due to suspected diabetes and had shared a ward with two children who had contracted malaria while in Burkina Faso.

“It’s not probable, almost impossible, to pass on the [parasite] from patient to patient,” Paternoster, in an interview with an international news agency, said it wasn’t probable – and almost impossible – to pass on the parasite from patient to patient. The only option they could think of is that the mosquito that carried the disease may have traveled to Italy and somehow survived.

How the child could have been infected still baffles them, he added.

Italy has been malaria-free since the 1950s, with most recorded cases linked to tourists who returned from countries where it is common.

Paternoster speculated that climate change may be to blame. He was quoted by an Italian daily describing the summer season as “a long and very hot” one.

The World Health Organization (WHO), on September 5, was hosting a meeting in Moscow to discuss how to keep Europe malaria-free. Zero cases of home-grown malaria were reported in Europe in 2015.

The WHO says Italy could be vulnerable to a return of malaria if mosquitoes are not properly controlled.

Malaria is caused by a parasite that has a complex life cycle dependent on both animals, including humans, and mosquitoes.